1. Field of the Invention
The invention relates generally to the field of roller cone drill bits used to drill boreholes through earth formations. More specifically, the invention relates for devices for excluding dust and debris from the bearings on roller cone bits which can extend the life of such bits.
2. Description of the Related Art
Roller cone bits are used in many applications to drill wellbores through earth formations. A typical roller cone bit includes one or more roller cones rotatably mounted on a bearing journal. The journal forms part of or is coupled to a xe2x80x9clegxe2x80x9d. The leg forms part of a bit body. The bit body includes a connection at one end, typically threaded, which connects the bit to a rotary power source, such as a downhole motor or a rotary drilling rig. The bit drills by the crushing and gouging action of cutting elements disposed about the surface of the roller cones as the bit is rotated about its axis by the rotary power source.
One application for roller cone drill bits is in the mining industry and the construction industry, where such bits are used to drill xe2x80x9cshot holesxe2x80x9d in rock for explosives to be inserted therein. Typical drilling rigs used to drill such shot holes include a compressed air source, which pumps air through the threaded connection into the bit. The air is used in some types of roller cone bits to clean and cool the bearings on which the roller cones revolve, and in all types of roller cone bits to cool the cutting elements and the cut rock face, and to lift rock cuttings out of the borehole as it is being drilled. The air which lifts the cuttings is generally discharged through one or more nozzles disposed in the bit body near the roller cones.
Some roller cone bits used in mining and construction shot hole applications include roller cone bearings which are not sealed. Part of the air pumped through the connection to clean the cuttings in these bits is diverted into the bearings through a passage drilled in each leg and bearing journal. Dust and debris which accumulates on the bearing journal during drilling is at least partially cleaned by the air flow. These bits typically do not have a very long bearing life, primarily due to the dust and debris which unavoidably enters and remains in the bearing. Some of the dust and debris which lodges in the bearings enters the bearings during xe2x80x9cpipe tripsxe2x80x9d. During a pipe trip, the drilling assembly is removed from the existing wellbore by uncoupling segments of drill pipe to which the bit is attached. During this time, the compressed air is turned off. Cuttings which have not been lifted out of the wellbore may migrate into the bit bearings.
An improvement to the air-cleaned bearing bit which is intended to deal with the problem of cuttings entering the bearings is described in U.S. Pat. No. 3,685,601 issued to Hollingshead. The bit described in this patent includes a backflow valve (BFV) disposed in a nozzle retaining socket. The BFV prevents dust and debris from entering the interior of the bit through the nozzles, particularly during pipe trips. Some of this dust and debris is believed to enter the bearings through the air channels in the legs of the bit. Using a BFV in air-cleaned, open bearing bits did not substantially improve the life of these bits, however.
Other air cleaned bits include seals between the roller cone and bearing journal. These bits show improved life as compared to unsealed bits. One such bit is described, for example, in published British patent no. GB 2019921 filed by Schumacher. The bit disclosed in this patent includes small air passages for keeping the bearing seals clean. Notably, the air passages in sealed bearing, air cleaned bits typically terminate in the seal area of the journal and not in the bearing area of the journal. Sealed bearing bits have an internal lubrication system for the bearings which is separated from the air used to clean and cool the bit.
A limitation to air-cleaned, sealed bearing bits known in the art is that the air passages are typically much smaller than those used in unsealed, air-cleaned bearing bits. The small passages are particularly susceptible to plugging with dust and debris. U.S. Pat. No. 4,981,182 issued to Dysart show a sealed bearing, air-cleaned bit which includes a porous gas restrictor at the opening of the cone mouth used to prevent dust and debris from entering the seal area during times when no air is supplied to the bit. This bit includes an air screen disposed at the top of the small air passages used to conduct air to the bearings. The air passages are subject to plugging when the air is turned off (such as during pipe trips) and dust and debris enter the interior of the bit.
U.S. Pat. No. 4,184,554 issued to Leverfelt describes an air bearing bit including a provision for excluding debris from the air channels in the bit which supply air to the bearings. Much of the debris exclusion system in the bit described in this patent is directed to separating water from the air stream, rather than excluding debris from the bearings. Further, the bearings on the bit shown in the Leverfelt ""554 patent are unsealed, and are subject to early failure due to dust and debris entering the bearings from the open cone mouth.
Yet another type of air cleaned bit is shown in U.S. patent applications Ser. Nos. 06/293,053, now abandoned, and 06/293,054, now abandoned, filed on Aug. 17, 1981 by Price et al. The bit shown in these applications includes an air pressure regulator to control the flow of air to air-cleaned bearings (which are unsealed) in a roller cone bit.
Another type of sealed-bearing, air-cleaned bit is sold under the trade name xe2x80x9cCHARGERxe2x80x9d by Smith International, Inc., Houston, TX, also the assignee of the present invention. A cross-section of one of these prior art bits is shown in FIG. 1. A bit body 10 is formed from one or more xe2x80x9clegsxe2x80x9d 15A, each of which includes a bearing journal 15 formed thereon. Each journal includes a roller cone (not shown) having a plurality of cutting elements thereon, rotatably mounted to the journal 15. The bit body 10 includes a threaded coupling 11 at one end for connection to drill pipe (not shown) in a conventional manner. The drill pipe (not shown) conducts compressed air to the bit. Each leg 15A includes an air channel 17 drilled from the seal area of the journal 15 to the interior 18 of the bit body 10. Some of the compressed air which enters the bit from the threaded connection 11 is diverted through the air channels 17 to keep the seal area free of dust and debris. The bit shown in FIG. 1 includes a backflow valve 13 disposed inside the area of the threaded connection 11. The backflow valve 13 only allows air (and dust/debris) movement in one direction, and therefore reduces the amount of dust and debris which enters the interior 18 of the bit. However, some dust and debris still can enter the bit during pipe trips. Such dust and debris has been known to plug one or more of the air channels 17. When one of the air channels 17 becomes plugged, the bearing seal associated with the plugged air channel cannot receive air. In this case, the seal is subject to early failure. Seal failure leads to rapid subsequent bearing failure in a sealed bearing bit.
It is desirable to provide a sealed bearing, air-cleaned bit which has reduced susceptibility to plugging of air passages used to keep the seals clean.
The invention is a roller cone drill bit which includes a bit body having at least one leg depending from the bit body. The at least one leg has a journal formed on its at a lower end. The bit body includes an air channel extending at least from an interior of the bit body out to the seal area of the journal. A roller cone is sealingly, rotatably mounted on the journal. The cone has a plurality of cutting elements disposed on it, The bit includes a backflow valve disposed in the bit body, so that an intake end of the air channel is disposed upstream of the backflow valve.
In one embodiment, the backflow valve is disposed in a connection end of the bit body, and the air channel includes a flow tube extending from the air passage in the bit body to the backflow valve so that its intake end is upstream of the backflow valve.
In another embodiment, the bit body includes a plurality of legs, each having a journal and roller cone rotatably, sealingly mounted on the journal. Each leg has an air passage extending from the seal area of the journal to the interior of the bit body so that its intake end extends upstream of a backflow valve disposed in the bit body. In one embodiment, each air passage includes a flow tube extending upstream of a backflow valve which is positioned in a connection end of the bit. One embodiment of the backflow valve is a spring loaded flapper valve.